1. Field of the Invention
The present invention relates to a personal handy phone system (PHS), and more particularly to linkage with a public telephone line.
2. Description of the Related Arts
In general, the PHS includes, as radio channel, four time slots each of which is constituted as 32 kbps ADPCM channel. As illustrated in FIG. 1, when the data sent from an external transmitter through the air is received by a transmitter/receiver 51 of TDMA/TDD system, and if the PHS channel is linked to two B channels and one D channel on an ISDN subscriber interface, the received data is separated on a time slot-by-time slot basis, taken out as received data for four channels such as B'1, B'2, B'3 and B'4, and inputted into 32 kbps data receiving registers (rr) 521.about.524. The outputs of the registers 521.about.524 are converted to 64 kbps .mu.-law PCM data B1.about.B4 by first converters (1st t) 541.about.544 each of which operates to convert 32 kbps ADPCM data into 64 kbps .mu.-law PCM data, and sent to 64 kbps data transmission registers (tr) 561.about.564. The tranmission registers 561.about.564 separate the respective inputs thereto into two sets of data (a set of B1 and B2; B3 and B4, for example) for connection to two B channels and one D channel, and transmit them to S interfaces 581 and 582, and the data are exchanged to respective ISDN subscriber lines.
On the other hand, the data sent from wired telephones are provided through the S interfaces 581 and 582 to second converters (2nd t) 551.about.554 each of which operates to convert 64 kbps .mu.-law PCM data into 32 kbps ADPCM data. The outputs of the second converters 551.about.554 are supplied through transmission registers 531.about.534 to the transmitter/receiver 51, and then transmitted into the air.
The B channel of ISDN subscriber interface has a transmission capability of 64 kbps. On the contrary, in the conventional technology wherein data has to be converted once into 64 kbps data in order to link the PHS data having a transmission speed of 32 kbps to the B channel of ISDN, the B channel has not been effectively utilized.
In addition, since there must be disposed the first and second converters for every four time slot within a base station, a large scale of circuit will be required for the system. Consequently, the whole facilities in the base station naturally become bulky.
According to the transmission control procedure in PHS now in effect, when data error occurs in a radio channel, one burst data including error data portion is to be discarded without performing error correction. However, in the case of ADPCM demodulation of voice data for PHS, only the differential data from the past data is concerned with the demodulation thereof. Thus, in the event that the received data disappears as the result of discarding of one burst data due to data error, it usually becomes impossible for the first converter to reproduce the voice data even when it thereafter receives normal data. To avoid this disadvantage, it is customarily practiced that the missing portion is generally filled with dummy data to interpolate the received data. What's more, since most of users desire to have more natural speech, highly sophisticated interpolation processing may often be utilized so as not to provide the users with unharmonious feeling. However, to carry out high grade interpolation processing, not only a scale of circuit and an external configulation or frame size, but also consumption current or current flow in a battery provided as power source become large, thus causing a cumbersome problem to be introduced in the design thereof. In the case of the prior art PHS, judgment as to whether or not the error of data has occured could be made only at the base station, so that the interpolation processing regarding the received data had to be made within the base station because no means for sending to ISDN exchange the information with regard to the occurence of troubles is provided.
As the sophistication of the base station grows more and more, the whole frame will also become bulk and heavy. An increase in the size of the whole frame prevents easy installation of the base station and give rise to problems in safety and the PHS which is to be constructed by use of microcells in the near future, and a business party must bear an increase in cost accompanied thereby.
As a specific system capable of effectively utilizing the B channel of ISDN, a time-shared exchange for multiplexing a plurality of low speed data channels and one signal channel is proposed in a paper, entitled "Investigation on 8.times.N kb/s multiple call linking system in ISDN exchange", The Institute of Electronics and Communication Engineers, Information Networks IN85-5. As described in the paper, effective utilization of the B channel for a plurality of low speed traffic calls can be accomplished by time sharing one data channel at a constant ratio for multiplexing. By way of example, there is Japanese Patent Laid-Open No. 215132/88, entitled "Multi-processing system for slow packet call". In this patent, an additional device is disposed which is linked to the time-shared exchange via an information channel and a call control signal line, and the division of packet corresponding to subchannels is carried out in the additional device.
Although a mere multiplexing may be accomplished by applying the prior art PHS to the above mentioned multuplex system as it is, it will be evident that various problems inherent to the prior art PHS may not be solved.